Ultra high speed time domain multiplexing (TDM) optical transmission systems in optical fibers require compact light emitting sources capable of generating optical short pulse trains in a picosecond/sub-picosecond range. General requirements for short pulse sources such as soliton sources are narrow pulse width, low time jitter and a continuously tunable repetition rate. For practical fiber optic systems, there are additional requirements of long-term reliability, small size and easy data encoding in the system application.
There are several known methods to generate short optical pulse trains. Complicated passive or active mode locking techniques are available for high speed optical pulse generation where pulses are generated at a fixed repetition rate determined by the roundtrip time of the laser resonator, e.g. D. J. Derickson et al., "Short pulse generation using multisegment mode-locked semiconductor lasers," IEEE J. Quantum Electron., Vol. 28, pp. 2186-2202, 1992. These techniques are sensitive to phase matching conditions and therefore difficult to build and maintain. Another method is gain switching of lasers which suffer from high time jitter. Pulse generation at repetition rates over 50 GHz is extremely difficult to achieve in this method because of limitations of the device modulation bandwidth and radio frequency supply as described, e.g. in publication by A. G. Weber, W. Ronghan, E. H. Bottcher, M. Schell and D. Bimberg, "Measurement and simulation of the turn-on delay time jitter in gain-switched semiconductor lasers," IEEE J. Quantum Electron., Vol. 28, pp. 441-445, 1992.
High repetition rate optical pulses can also be generated using a dual wavelength light source as described, e.g. in publication P. V. Mamyshev, S. V. Chernikov and E. M. Dianov, "Generation of Fundamental soliton trains for high-bit-rate optical fiber communication lines," IEEE J. Quantum Electron., vol. 27, pp. 2347-2355, 1991. Two wavelengths emitted by two lasers are mixed to form a high frequency sinusoidal signal which is sent though an optical combiner and optical amplifier followed by a nonlinear fiber. As a result the sinusoidal signal is compressed into a train of optical pulses. The common approach of dual wavelength light sources is to use two discrete lasers, which is complex and suffers from the long term stability issue. Dual wavelength operation can also be accomplished by selecting the appropriate phase modulation sidebands from an externally phase modulated light source, e.g. P. V. Mamyshev, "Dual-wavelength source of high-repetition-rate, transform-limited optical pulses for soliton transmission" Opt. Lett., Vol. 19, pp. 2074-2076, 1994. This method requires high frequency modulation and optical filters. In order to make this method more practical two solitary laser diodes are usually used to generate the sinusoidal beat signal. Unfortunately, frequency variations of each laser are subject to both thermal and mechanical fluctuations which result in beat signal frequency fluctuations. Phase noise of both lasers also contribute to the jitter of the beat signal significantly. The control of the polarization from each laser output and the effort to align and maintain them is also a practical issue that decreases the system performance. Therefore, the resulting signal performance is not satisfactory and practical use of such a configuration in commercial ultra high speed applications is in question.
Accordingly, there is a need in the industry for a practical, compact and reliable optical source of continuously tuning high repetition rate short optical pulses which is suitable for optical transmission systems and high speed optical signal processing.